The present invention relates to golf balls having ionomeric covers, and more particularly to golf balls having covers formed from acrylate ester-containing ionic copolymers.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E. I. DuPont deNemours and Company under the trademark xe2x80x9cSurlyn(copyright)xe2x80x9d and by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademark xe2x80x9cEscor(copyright)xe2x80x9d and the tradename xe2x80x9clotekxe2x80x9d, have become the materials of choice for the construction of golf ball covers over the traditional xe2x80x9cbalataxe2x80x9d (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability necessary for repetitive play.
Ionomeric resins are ionic copolymers of an olefin such as ethylene and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc., for golf ball cover construction over balata. However, the advantages gained in increased durability have been offset to some degree by the decreases produced in playability. This is because the durable ionomeric resins tend to be very hard when utilized for golf ball cover construction, and thus lack the degree of softness required to impart the spin necessary to control the ball in flight.
As a result, while there are currently more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcements, etc., a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the improved impact resistance and carrying distance properties produced by the xe2x80x9chardxe2x80x9d ionomeric resins, but also the playability (i.e. xe2x80x9cspinxe2x80x9d) characteristics previously associated with the xe2x80x9csoftxe2x80x9d balata covers, properties which are still desired by the more skilled golfer.
In various attempts to produce a durable, high spin ionomeric golf ball, the golfing industry has blended the hard ionomeric resins with a number of softer ionomeric resins. U.S. Pat. Nos. 4,884,814 and 5,120,791 are directed to cover compositions containing blends of hard and soft ionomeric resins. The hard copolymers typically are made from an olefin and an unsaturated carboxylic acid. The soft copolymers are generally made from an olefin, an unsaturated carboxylic acid, and an acrylate ester. It has been found that golf ball covers formed from hard-soft ionomer blends tend to become scuffed more readily than covers made of hard ionomer alone. It would be useful to develop a golf ball having a combination of softness and durability which is better than the softness-durability combination of a golf ball cover made from a hard-soft ionomer blend.
An object of the invention is to provide a golf ball with a soft cover which has good scuff resistance.
Yet another object of the invention is to provide a golf ball having a favorable combination of spin rate and durability.
A further object of the invention is to provide a golf ball having a soft cover made from a cover material which is blended with minimal mixing difficulties.
Another object of the invention is to provide a method of making a golf ball which has a soft cover with good scuff resistance and cut resistance.
Yet another object of the invention is to provide a method of making a durable golf ball with a relatively high spin rate.
Other objects will be in part obvious and in part pointed out more in detail hereafter.
The invention in a preferred form is a golf ball comprising a core and a scuff-resistant cover. The cover comprises an ionomeric resin having more than 90 weight % of one or more acrylate ester-containing ionic copolymers. The one or more acrylate ester-containing ionic copolymers are each formed from (a) an olefin having 2 to 8 carbon atoms, (b) an unsaturated monomer of the acrylate ester class having from 1 to 21 carbon atoms, and (c) an acid which includes at least one member selected from the group consisting of xcex1, xcex2-ethylenically unsaturated mono- or dicarboxylic acids with a portion of the acid groups being neutralized with cations. The one or more acrylate ester-containing ionic copolymers have an overall Shore D hardness of 40-64. The resulting golf ball has a coefficient of restitution of at least 0.770.
The one or more acrylate ester-containing ionic copolymers preferably are terpolymers. In each copolymer, the olefin preferably is an alpha olefin, and the acid preferably is acrylic acid. The overall cover preferably has a Shore D hardness of no more than about 64, and more preferably about 40 to 64.
The one or more acrylate ester-containing ionic copolymers typically have a degree of neutralization of the acid-groups in the range of about 10-100%. In a preferred form of the invention, the covers have a scuff resistance rating of 3.0 or better when subjected to the Golf Ball Cover Scuff Test which is described below.
In a particularly preferred form of the invention, the cover comprises an ionomeric resin having at least 95 weight % of one or more acrylate ester-containing ionic copolymers. The cover preferably contains at least 90 weight % ionomeric resin. Each of the acrylate ester-containing copolymers preferably comprises ethylene, at least one acid selected from the group consisting of acrylic acid, maleic acid, fumaric acid, itaconic acid, methacrylic acid, and half-esters of maleic, fumaric and itaconic acids, and at least one comonomer selected from the group consisting of methyl, ethyl, n-propyl, n-butyl, n-octyl, 2-ethylhexyl, and 2-methoxyethyl-1-acrylates.
Another preferred form of the invention is a method of making a golf ball. The method comprises the steps of obtaining a golf ball core and forming a scuff-resistant cover over the core. The cover comprises an ionomeric resin having more than 90 weight % of one or more acrylate ester-containing ionic copolymers formed from (a) an olefin having 2 to 8 carbon atoms, (b) an unsaturated monomer of the acrylate ester class having from 1 to 21 carbon atoms, and (c) an acid which is selected from the group consisting of alpha, beta-ethylenically unsaturated mono- or dicarboxylic acids with a portion of the acid groups being neutralized with cations. The one or more acrylate ester-containing ionic copolymers preferably have a Shore D hardness of about 40-64.
The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others and the article possessing the features, properties, and the relation of elements exemplified in the following detailed disclosure.
The present invention overcomes mixing difficulties associated with the preparation of soft-covered ionomeric golf balls by forming the cover of soft ionomeric golf balls from a cover composition in which the ionomer has more than 90 weight % of an acrylate ester-containing ionic copolymer or blend of acrylate ester-containing ionic copolymers. Furthermore, the invention provides golf ball covers having a favorable combination of durability and spin rate.
The one or more acrylate ester-containing ionic copolymers to be used in forming the cover of the inventive golf ball each contain an olefin, an acrylate ester, and an acid. In a blend of two or more acrylate ester-containing ionic copolymers, each copolymer may contain the same or a different olefin, acrylate ester and acid than are contained in the other copolymers. Preferably, the acrylate ester-containing ionic copolymer or copolymers are terpolymers, but additional monomers can be combined into the copolymers if the monomers do not substantially reduce the scuff resistance or other good playability properties of the cover.
For a given copolymer, the olefin is selected from the group consisting of olefins having 2 to 8 carbon atoms, including, as non-limiting examples, ethylene, propylene, butene-1, hexene-1 and the like. Preferably the olefin is ethylene.
The acrylate ester is an unsaturated monomer having from 1 to 21 carbon atoms which serves as a softening comonomer. The acrylate ester preferably is methyl, ethyl, n-propyl, n-butyl, n-octyl, 2-ethylhexyl, or 2-methoxyethyl 1-acrylate, and most preferably is methyl acrylate or n-butyl acrylate. Another suitable type of softening comonomer is an alkyl vinyl ether selected from the group consisting of n-butyl, n-hexyl, 2-ethylhexyl, and 2-methoxyethyl vinyl ethers.
The acid is a mono- or dicarboxylic acid and preferably is selected from the group consisting of methacrylic, acrylic, ethacrylic, xcex1-chloroacrylic, crotonic, maleic, fumaric, and itaconic acid, or the like, and half esters of maleic, fumaric and itaconic acid, or the like. The acid group of the copolymer is 10-100% neutralized with any suitable cation, for example, zinc, sodium, magnesium, lithium, potassium, calcium, manganese, nickel, chromium, tin, aluminum, or the like. It has been found that particularly good results are obtained when the neutralization level is about 50-100%.
The one or more acrylate ester-containing ionic copolymers each has an individual Shore D hardness of about 5-64. The overall Shore D hardness of the acrylate ester-containing ionic copolymer or blend of acrylate ester-containing ionic copolymers is 40-64. It is preferred that the overall Shore D hardness of the acrylate ester-containing ionic copolymer or blend is in the range of 50-64 in order to impart particularly good playability characteristics to the ball. It has been found that excellent results can be obtained when the Shore D hardness of the acrylate ester-containing ionic copolymer or acrylate ester-containing ionic copolymer blend is in the range of 52-54 for a soft covered golf ball or 62-64 for a somewhat harder ball.
The cover of the invention is formed over a core to result in a golf ball having a coefficient of restitution of at least 0.770, more preferably at least 0.780, and most preferably at least 0.790. The coefficient of restitution of the ball will depend upon the properties of both the core and the cover. The cover of the present invention is adapted for use with any type of golf ball core.
The acrylate ester-containing ionic copolymer or copolymers used in the golf ball of the invention can be obtained by neutralizing commercially available acrylate ester-containing acid copolymers such as polyethylene-methyl acrylate-acrylic acid terpolymers, including ESCOR ATX (Exxon Chemical Company) or poly (ethylene-butyl acrylate-methacrylic acid) terpolymers, including NUCREL (DuPont Chemical Company). Particularly preferred commercially available materials include ATX 320, ATX 325, ATX 310, ATX 350, and blends of these materials with NUCREL 010 and NUCREL 035. The acid groups of these materials and blends are neutralized with one or more of various cation salts including zinc, sodium, magnesium, lithium, potassium, calcium, manganese, nickel, etc. The degree of neutralization ranges from 10-100%. Generally, a higher degree of neutralization results in a harder and tougher cover material. The properties of non-limiting examples of commercially available un-neutralized acid terpolymers which can be used to form the golf ball covers of the invention are provided below in Table 1.
The ionomer resins used to form the golf balls of the invention are produced by reacting the acrylate ester-containing acid copolymer with various amounts of the metal cation salts at a temperature above the crystalline melting point of the copolymer, such as a temperature from about 200xc2x0 F. to about 500xc2x0 F., preferably from about 250xc2x0 F. to about 350xc2x0 F., under high shear conditions at a pressure of from about 100 psi to 10,000 psi. Other well known blending techniques may also be used. The amount of metal cation salt utilized to produce the neutralized ionic copolymers is the quantity which provides a sufficient amount of the metal cations to neutralize the desired percentage of the carboxylic acid groups in the high acid copolymer. When two or more different copolymers are to be used, the copolymers can be blended before or after neutralization. Generally, it is preferable to blend the copolymers before they are neutralized to provide for optimal mixing.
The compatibility of the acrylate ester-containing copolymers with each other in a copolymer blend produces a golf ball having a surprisingly good scuff resistance for a given cover hardness. The golf ball according to the invention has a scuff resistance of no higher than 3.0. It is preferred that the golf ball has a scuff resistance of no higher than about 2.5 to ensure that the golf ball is scuff resistant when used in conjunction with a variety of types of clubs, including sharp-grooved irons, which are particularly inclined to result in scuffing of golf ball covers. The best results according to the invention are obtained when the cover has a scuff resistance of no more than about 2.0. The scuff resistance test is described in detail below.
The golf ball of the invention has a spin rate of 7,500 or more. It is particularly preferred that the golf ball of the invention have a spin rate of 8,000 or more in order to provide experienced players with excellent spin characteristics. In a particularly preferred form of the invention, the golf ball has a spin rate of at least 8,500. These spin rates are surprisingly high given the good scuff and cut resistance ratings of the ionomeric cover material. The test for spin rate is described below.
Additional materials may also be added to the ionic cover compositions of the present invention as long as they do not substantially reduce the playability properties of the ball. Such materials include dyes (for example, Ultramarine Blue sold by Whitaker, Clark, and Daniels of South Plainsfield, N.J.) (see U.S. Pat. No. 4,679,795), pigments such as titanium dioxide, zinc oxide, barium sulfate and zinc sulfate; UV absorbers; antioxidants; antistatic agents; and stabilizers. Moreover, the cover compositions of the present invention may also contain softening agents such as those disclosed in U.S. Pat. Nos. 5,312,857 and 5,306,760, including plasticizers, metal stearates, processing acids, etc., and reinforcing materials such as glass fibers and inorganic fillers, as long as the desired properties produced by the golf ball covers of the invention are not impaired. Furthermore, non-ionomeric cover materials such as e.g., metallocene catalyzed polyolefins, including EXACT materials available from EXXON, can be blended with the terpolymers as long as a favorable combination of scuff resistance, COR, compression and spin rate is obtained.
When a blend of two or more acrylate ester-containing ionic copolymers is to be used, the cover compositions of the present invention may be produced according to conventional melt blending procedures. Generally, the copolymer resins are blended in a Banbury type mixer, two-roll mill, or extruder prior to neutralization. After blending, neutralization then occurs in the melt or molten state in the Banbury mixer. Mixing problems are minimal because preferably more than 91 wt %, and more preferably at least 95 wt % of the ionic copolymers in the mixture contain acrylate esters, and in this respect, most of the polymer chains in the mixture are similar to each other. The blended composition is then formed into slabs, pellets, etc., and maintained in such a state until molding is desired. If necessary, further additives such as inorganic fillers, antioxidants, stabilizers, processing aids, etc., may be added and uniformly mixed before initiation of the molding process. In one embodiment of the invention, a masterbatch of non-acrylate ester-containing ionomer with pigments and other additives incorporated therein is mixed with the acrylate ester-containing copolymers in a ratio of about 1-7 weight % masterbatch and 93-99 weight % acrylate ester-containing copolymer.
The golf balls of the present invention can be produced by molding processes which include but are not limited to those which are currently well known in the golf ball art. For example, the golf balls can be produced by injection molding or compression molding the novel cover compositions around a wound or solid molded core to produce a golf ball having a diameter of about 1.680-1.80 inches and typically but not necessarily weighing about 1.620 ounces. The standards for both the minimum diameter and maximum weight of the balls are established by the United States Golf Association (U.S.G.A.).
The cores of the inventive golf balls typically have a coefficient of restitution of about 0.775 and a PGA compression of about 110. The term xe2x80x9csolid coresxe2x80x9d as used herein refers not only to one piece cores but also to those cores having a separate solid layer beneath the cover and over the core. When the golf ball of the invention has a solid core, this core can be compression molded from a slug of uncured or lightly cured elastomer composition comprising a high cis content polybutadiene and a metal salt of an xcex1, xcex2, ethylenically unsaturated carboxylic acid such as zinc mono- or diacrylate or methacrylate. To achieve higher coefficients of restitution and/or to increase hardness in the core, the manufacturer may include a small amount of a metal oxide such as zinc oxide. In addition, larger amounts of metal oxide than are needed to achieve the desired coefficient may be included in order to increase the core weight so that the finished ball more closely approaches the U.S.G.A. upper weight limit of 1.620 ounces. Non-limiting examples of other materials which may be used in the core composition including compatible rubbers or ionomers, and low molecular weight fatty acids such as stearic acid. Free radical initiator catalysts such as peroxides are admixed with the core composition so that on the application of heat and pressure, a curing or cross-linking reaction takes place.
Wound cores are generally produced by winding a very large elastic thread around a solid or liquid filled balloon center. The elastic thread is wound around the center to produce a finished core of about 1.4 to 1.6 inches in diameter, generally. Since the core material is not an integral part of the present invention, a detailed discussion concerning the specific types of core materials which may be utilized with the cover compositions of the invention are not specifically set forth herein.
As indicated, the golf balls of the present invention may be produced by forming covers consisting of the above-described compositions around cores by molding processes. For example, in compression molding, the cover composition is formed via injection at e.g. about 380xc2x0 F. to about 450xc2x0 F. into smooth surfaced hemispherical shells which are then positioned around the core in a dimpled golf ball mold and subjected to compression molding at e.g. 200xc2x0-300xc2x0 F. for 2-10 minutes, followed by cooling at 50xc2x0-70xc2x0 F. for 2-10 minutes, to fuse the shells together to form an unitary ball. In one type of injection molding process, the cover composition is injected directly around the core placed in the center of a golf ball mold for a period of time at a mold temperature of from 50xc2x0 to about 100xc2x0 F. After molding, the golf balls produced may undergo various further processing steps such as buffing, painting, and marking.